.@Billbrowder has been arrested in Spain. I interviewed him in London last week. He never holds back on his criticism of Mr Putin. Russia requested an Interpol arrest warrant a few months ago for charges linked to corruption and murder all of which he denies. pic.twitter.com/ymrAShXryQ

Mr Browder tweeted that "ironically", the reason he was in Madrid was to "give evidence to senior Spanish anti-Russian mafia prosecutor Jose Grinda about the huge amount of money from the Magnitsky case that flowed to Spain".